Brake System and Method of Control with Air Gap Estimation

ABSTRACT

A brake system and a method of control. An air gap between a brake friction member and a brake pad assembly may be indirectly estimated. The air gap may be adjusted when the estimated air gap exceeds a desired air gap.

TECHNICAL FIELD

This patent application relates to a brake system and method of controlin which an air gap between a brake friction member and a brake padassembly may be estimated.

BACKGROUND

An air gap detector for use in antilocking brake systems is disclosed inU.S. Pat. No. 5,432,442.

SUMMARY

In at least one embodiment, a method of controlling a brake system isprovided. The method may include indirectly estimating an air gapbetween a brake friction member and a brake pad assembly based on asignal from a position sensor that detects cycling of the frictionbrake. The air gap may be adjusted when an estimated air gap exceeds adesired air gap.

In at least one embodiment, a method of controlling a brake system isprovided. The method may include cycling a friction brake by actuating abrake pad assembly from a retracted position into engagement with abrake friction member and releasing the brake pad assembly such that thebrake pad assembly disengages the brake friction member and moves towardthe retracted position. An air gap between the brake friction member andthe brake pad assembly may be indirectly estimated based on a signalfrom a position sensor that may be indicative of actuation of a brakepedal. The air gap may be adjusted when an estimated air gap exceeds adesired air gap.

In at least one embodiment, a brake system is provided. The brake systemmay include a brake friction member, a brake pad assembly, an actuatorsubsystem, and a position sensor. The brake pad assembly may beconfigured to move between a retracted position and an extendedposition. The brake pad assembly may not engage the brake frictionmember when in the retracted position. The brake pad assembly may engagethe brake friction member when in the extended position. The actuatorsubsystem may include an operating shaft that may rotate when the brakepad assembly moves between the retracted and extended positions. Theposition sensor may detect rotation of the operating shaft. An air gapbetween the brake pad assembly and the brake friction member may beestimated based on a signal from the position sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic of a brake system.

FIGS. 2 and 3 are flowcharts associated with a method of controlling abrake system.

FIGS. 4-7 are exemplary plots that help illustrate method steps.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

Referring to FIG. 1, an exemplary brake system 10 is shown. The brakesystem 10 may be provided with a vehicle, such as a motor vehicle like atruck, farm equipment, or military transport or weaponry vehicle. Thevehicle may include a trailer for transporting cargo in one or moreembodiments.

The brake system 10 may be configured to slow or inhibit rotation of atleast one associated wheel assembly. The brake system 10 may include aset of friction brakes 12, an actuator subsystem 14, and a controlsystem 16.

A friction brake 12, which may also be called a foundation brake, may bedisposed proximate a wheel assembly and may be configured to slowrotation of the wheel assembly. Multiple friction brakes 12 may beprovided with the vehicle and may be controlled by the control system16. The friction brake 12 may have any suitable configuration. Forexample, a friction brake 12 may be configured as a disc brake. Thefriction brake 12 may include a brake friction member 20 and at leastone brake pad assembly 22.

The brake friction member 20 may be connected to a wheel hub. As such,the brake friction member 20 may rotate with a wheel assembly and withrespect to a brake pad assembly 22 when braking is not requested. In adisc brake configuration, the brake friction member 20 may be configuredas a rotor, which is also known as a brake disc.

In a disc brake configuration, the brake friction member 20 may extendinto an opening in a carrier (not shown). The carrier may be fixedlymounted to the vehicle and may receive and/or support inboard andoutboard brake pad assemblies 22. As such, the carrier may straddle thebrake friction member 20 and help position the brake pad assemblies 22on opposite sides of the brake friction member 20.

The brake pad assembly 22 may engage the brake friction member 20 whenbraking is requested or commanded and exert frictional force against thebrake friction member 20 to retard or slow rotation of an associatedwheel assembly. In a disc brake configuration, inboard and outboardbrake pad assemblies 22 may be disposed on opposite sides of the brakefriction member 20 and may be configured to engage opposite sides of thebrake friction member 20 to slow the rotation of a vehicle wheel. Thebrake pad assemblies 22 may be received in a caliper housing (not shown)that may be movably disposed on the carrier. More specifically, thecaliper housing may be slidably disposed on a pair of slide pins thatmay be fixedly disposed on the carrier. The caliper housing may receiveat least a portion of the actuator subsystem 14, which may actuate thebrake pad assemblies 22 into engagement with the brake friction member20. For example, the actuator subsystem 14 may include one or morepistons that may actuate the inboard brake pad assembly 22 toward thebrake friction member 20 and move the caliper housing to actuate theoutboard brake pad assembly 22 toward the rotor as will be discussed inmore detail below.

The brake pad assembly 22 may include a backing plate 30 and a frictionmaterial 32.

The backing plate 30 may be a structural member of a brake pad assembly22. The backing plate 30 may be made of any suitable material, such as ametal or metal alloy.

The friction material 32, which may also be called a brake lining, maybe disposed on the backing plate 30. The friction material 32 may facetoward the brake friction member 20 and may engage the brake frictionmember 20 during vehicle braking. In addition, the friction material 32may be spaced apart from and may not engage the brake friction member 20when the friction brake 12 is not being applied. The clearance ordistance between the friction material 32 and the brake friction member20 when the friction brake 12 is not being applied may be referred to asan air gap 34 or running clearance. No clearance or zero clearance maybe present between the friction material 32 and the brake frictionmember 20 when the friction brake 12 is applied.

The actuator subsystem 14 may be configured to actuate a brake padassembly 22 between a retracted position and an extended position. Theretracted position may also be referred to as an initial position orrest position. The brake pad assembly 22 may be spaced apart from thebrake friction member 20 and may be stationary when in the retractedposition. As such, a brake pad assembly 22 may be in the retractedposition when the friction brake 12 is not being applied. The brake padassembly 22 and more specifically the friction material 32 may engagethe brake friction member 20 when in the extended position.

The actuator subsystem 14 may be disposed proximate or provided with thecaliper housing. The actuator subsystem 14 may have any suitableconfiguration. For example, the actuator subsystem 14 may have apneumatic, hydraulic, electrical, or electromechanical configuration, orcombinations thereof as are known by those skilled in the art. In thesimplified schematic shown in FIG. 1, the actuator subsystem 14 has apneumatic configuration. In such a configuration, the actuator subsystem14 may include an actuator 40, an actuation mechanism 42, and an air gapadjustment mechanism 44.

The actuator 40 may be configured to exert force that may be amplifiedand transmitted to actuate a brake pad assembly 22 toward the brakefriction member 20. The actuator 40 may have any suitable configuration.For example, the actuator 40 may be a linear actuator, such as apneumatic air chamber or pneumatic cylinder in one or more embodiments.In at least one embodiment, the actuator 40 may include an actuator rod50 that may move linearly between a first position (return position) anda second position (advanced position). The actuator 40 may be controlledby the control system 16 and may actuate a brake pad assembly 22 inresponse to a brake command as will be discussed in more detail below.

The actuation mechanism 42 may transmit force exerted by the actuator 40to the brake pad assemblies 22. For example, the actuator mechanism 42may include various mechanical components, such as linkages, shafts,bearings, rollers, springs, and one or more pistons that may cooperateto transmit force to actuate the brake pad assemblies 22 between theretracted and extended positions in a manner known by those skilled inthe art. For illustration purposes, a simplified example of an actuationmechanism 42 is shown in FIG. 1 that includes an operating shaft 60, alinkage 62, a roller 64, and an actuator unit 66.

The actuator 40 may be operatively connected to an operating shaft 60.In FIG. 1, the actuator 40 is indirectly connected to the operatingshaft 60 via the linkage 62, which may also be called an operatinglever. The linkage 62 may be coupled to the actuator rod 50. As such,extending the actuator rod 50 may cause the operating shaft 60 to rotateabout an axis 68 due to interaction with a roller 64. The roller 64 maytransmit force to the brake pad assemblies 22 via the actuator unit 66,which may include more pistons in a manner known by those skilled in theart.

Movement of the actuator 40 from the first position to the secondposition may cause the actuator rod 50 to move left from the positionshown in FIG. 1. Movement of the actuator rod 50 may transmit force tothe operating shaft 60 via the linkage 62 and may cause the operatingshaft 60 to rotate counterclockwise about the axis 68 and/or roller 64from the perspective shown. Force may then be transmitted to actuate abrake pad assembly 22 from the retracted position to the extendedposition by the actuator unit 66. In a floating caliper configuration,an inboard brake pad assembly 22 may engage the brake friction member 20before and outboard brake pad assembly 22. A reaction force may betransmitted to the outboard brake pad assembly 22 to pull the outboardbrake pad assembly 22 against the brake friction member 20 via movementof the floating caliper housing when the inboard brake pad assembly 22has engaged the brake friction member 20. As such, the inboard andoutboard brake pad assemblies 22 may cooperate to engage and brake orinhibit rotation of the brake friction member 20. The actuationmechanism components may return to their initial positions, or startingpositions when a brake command ends and the actuator 40 is permitted tomove from the second position to the first position (e.g., the actuatorrod 50 is permitted to back to its initial position). As such, theoperating shaft 60 may rotate in a first direction when a brake pedal isactuated and may rotate in a second direction disposed opposite thefirst direction when the brake pedal is released.

The air gap adjustment mechanism 44 may be provided to adjust the airgap to compensate for friction material wear or air gap or clearancechanges that may be due to temperature change or other factors. In atleast one embodiment, the air gap adjustment mechanism 44 may include anadjustment motor 70. The adjustment motor 70 may have any suitableconfiguration. For example, the adjustment motor 70 may be an electricmotor and may include an encoder that may be configured to measuredisplacement or rotation of a motor shaft to facilitate monitoring andcontrol of the adjustment motor 70. The adjustment motor 70 may beconnected to components that may establish or set the retracted positionor rest position of a brake pad assembly 22. For example, the adjustmentmotor 70 may be operatively connected to a tappet that may be providedwith or associated with the actuator unit 66. In at least oneembodiment, the tappet may receive the piston of the actuator unit 66and may have threads that interact with corresponding threads on thepiston to permit or inhibit relative rotation to adjust the air gap in amanner known by those skilled the art. Operation of the adjustment motor70 may be controlled by the control system 16.

The control system 16 may be configured to monitor and/or controloperation of the brake system 10. The control system 16 may include oneor more control modules or controllers 80 that may be provided tomonitor and control various components. For simplicity, a singlecontroller is shown in FIG. 1; however, it is contemplated that multiplecontrol modules or controllers or a distributed control architecture maybe provided. The controller 80 may monitor and control the actuatorsubsystem 14, air gap adjustment mechanism 44 and its adjustment motor70, and the amount of brake torque provided by the friction brakes 12.In addition, the controller 80 may also process input signals or datafrom various input devices or sensors such as a brake pedal sensor 82and an actuation mechanism position sensor 84.

A brake pedal sensor 82 may be provided to detect a braking command or abrake torque command that may be provided by a vehicle driver or vehicleoperator. For example, the brake pedal sensor 82 may detect the positionof a brake pedal 90 or the position or operating state of a componentthat may be connected to or operated by a brake pedal, such as a treadlevalve that may modulate a control fluid pressure that may be provided toa relay valve that may control the supply of fluid to one or morefriction brakes 12 or friction brake actuators. Alternatively, the brakepedal sensor 82 may be configured as a pressure sensor that may detectfluid pressure that may directly or indirectly control the actuator 40or braking of the vehicle. The detected position of the brake pedaland/or detected pressure may be used to control the brake torqueprovided by the brake system 10. For example, depending on theconfiguration of the brake system 10 the controller 80 may controloperation of a valve that controls fluid pressure provided to a frictionbrake 12, a brake pump that pressurizes fluid, and/or an electric motorthat may actuate a brake pad assembly 22. The amount of brake torqueprovided by the brake system 10 may be proportional to a detected angleof motion or amount of actuation of the brake pedal 90 or other braketorque command input device.

The actuation mechanism position sensor 84, which is also referred to asa position sensor, may detect rotation of the operating shaft 60. Assuch, the actuation mechanism position sensor 84 may provide a signal ordata that may be indicative of rotation of the operating shaft 60 aboutthe axis 68. Operating shaft rotation may indirectly detect or providedata indicative of the width or size of the air gap 34 or distance ofbrake pad assembly travel between the retracted position and extendedposition. The actuation mechanism position sensor 84 may be of anysuitable type. For example, the actuation mechanism position sensor 84may be an encoder.

Referring to FIGS. 2 and 3, flowcharts illustrating a method ofcontrolling the brake system 10 are shown. As will be appreciated by oneof ordinary skill in the art, the flowchart may represent control logicwhich may be implemented or affected in hardware, software, or acombination of hardware and software. For example, the various functionsmay be affected by a programmed microprocessor. The control logic may beimplemented using any of a number of known programming and processingtechniques or strategies and is not limited to the order or sequenceillustrated. For instance, interrupt or event-driven processing may beemployed in real-time control applications rather than a purelysequential strategy as illustrated. Likewise, parallel processing,multitasking, or multi-threaded systems and methods may be used.

Control logic may be independent of the particular programming language,operating system, processor, or circuitry used to develop and/orimplement the control logic illustrated. Likewise, depending upon theparticular programming language and processing strategy, variousfunctions may be performed in the sequence illustrated, at substantiallythe same time, or in a different sequence while accomplishing the methodof control. The illustrated functions may be modified, or in some casesomitted, without departing from the scope of the present invention. Inat least one embodiment, a method may be executed by the controller 80and may be implemented as a closed loop control system.

As an overview, the method may indirectly estimate the size or width ofthe air gap 34 between the brake friction member 20 and a brake padassembly 22 and may control or adjust the air gap as appropriate. Thewidth of the air gap 34 may not be directly detected by a sensor orother device that may directly measure the distance from the brakefriction member 20 and a brake pad assembly 22 when the brake padassembly 22 is in the retracted position. Indirect estimation of the airgap may be affected by various factors that may lead to inaccurate airgap estimates. For example, indirect estimation of the air gap may beaffected by multiple factors associated with the characteristics of theactuator subsystem 14 (wear, component expansion, component contraction,tolerance variations, vibrations, noise, position sensor bias and drift,etc.), which may be further affected by environmental factors (e.g.,temperature which may cause component expansion or contraction,contaminants, etc.). Moreover, noise and disturbance during air gapdetection, nonlinearity of the position sensor signal, and time delay inair gap adjustment may also influence air gap estimates. The methoddiscussed below may help provide more accurate air gap estimates andcontrol of the air gap.

Referring to FIG. 2, a flowchart that overviews the method is shown.

At block 100, the method may cycle the friction brake and obtain data ora signal from the actuation mechanism position sensor 84. The frictionbrake may be cycled when vehicle braking is desired or requested. Arequest for braking may be based on data or a signal from the brakepedal sensor 82. A request for braking may also be automaticallyinitiated or automatically requested rather than manually requested. Forexample, braking of the vehicle may be automatically initiated by anadaptive cruise control system or pre-impact collision avoidance system.An adaptive cruise control system may automatically adjust vehicle speedin response to the proximity and/or relative speed of another vehicle.As such, an adaptive cruise control system may brake the vehicle toadjust proximity and/or vehicle speed. A pre-impact collision avoidancesystem may be configured to detect a potential collision or impact withthe vehicle before it occurs and brake or slow the vehicle to avoid acollision. Cycling the friction brake may include actuating one or morebrake pad assemblies 22 from the retracted position to the extendedposition (i.e., into engagement with the brake friction member 20) andreleasing the brake pad assembly 22 such that the brake pad assembly 22disengages the brake friction member 20 and moves back to or toward theretracted position. The actuation mechanism position sensor 84 maydetect rotation of the operating shaft 60 during cycling of the frictionbrake 12. As such, a signal or data from the actuation mechanismposition sensor 84 may detect movement or the absence of movement of abrake pad assembly 22. An example of an exemplary signal from theactuation mechanism position sensor 84 is shown in FIG. 4 and will bediscussed in more detail below with the flowchart in FIG. 3.

At block 102, the air gap (i.e., air gap size or width) may beestimated. The size or width of the air gap 34 may be estimated based onthe signal or data from the actuation mechanism position sensor 84. Theestimated size or width of the air gap 34 may be referred to as theestimated air gap.

At block 104, the estimated air gap is compared to a desired air gap,which may also be referred to as a threshold air gap. The estimated airgap may be based on the signal from the actuation mechanism positionsensor 84 as previously discussed. The desired air gap may be apredetermined value and may be based on the configuration of the brakesystem and/or vehicle development testing. The desired air gap may beindicative of an air gap width or distance beyond which the brake padassembly 22 and its friction material 32 is not sufficiently close tothe brake friction member 20 when in the retracted position. Inaddition, the desired air gap may be indicative of an air gap width ordistance at which the brake pad assembly 22 and its friction material 32is too close to the brake friction member 20 when in the retractedposition. The desired air gap may be expressed as a single value but mayincorporate a tolerance range. For example, the desired air gap may beexpressed as a value like 0.6 mm, but may have a tolerance of +/−0.05mm, which thereby expresses the desired air gap as a range from 0.55 mmto 0.65 mm. Thus, the desired air gap may be a range rather than asingle absolute value. If the estimated air gap is sufficiently close tothe desired air gap, then the method may continue at block 106. If theestimated air gap is not sufficiently close to the desired air gap, thenthe method may continue at block 108.

At block 106, the air gap may not be adjusted. The air gap may not beadjusted since the estimated air gap indicates that the frictionmaterial 32 of at least one brake pad assembly 22 is sufficiently closeto the brake friction member 20 when in the retracted position toprovide a desired braking performance. Likewise, the air gap may not beadjusted when the estimated air gap indicates that the friction material32 of at least one brake pad assembly 22 is sufficiently distant or nottoo close to the brake friction member 20 when in the retractedposition. As such, the retracted position may not be adjusted.

At block 108, the air gap may be adjusted. For example, the air gap maybe reduced by operating the air gap adjustment mechanism 44 to move oneor more brake pad assemblies 22 closer to the brake friction member 20when the estimated air gap is greater than the desired air gap. As such,the retracted position of a brake pad assembly 22 may be changed suchthat the brake pad assembly 22 is moved closer to the brake frictionmember 20 than the previous retracted position or before making the airgap adjustment. Similarly, the air gap may be increased by operating theair gap adjustment mechanism 44 to move one or more brake pad assemblies22 further from the brake friction member 20 when the estimated air gapis less than the desired air gap. As such, the retracted position of abrake pad assembly 22 may be changed such that the brake pad assembly 22is moved further from the brake friction member 20 than the previousretracted position or before making the air gap adjustment. In at leastone embodiment, the air gap adjustment may be initiated or may occurduring release of the brake pad assembly 22, or while the brake padassembly 22 is moving from the extended position toward the retractedposition. The air gap 34 may be adjusted by operating the adjustmentmotor 70 to actuate the air gap adjustment mechanism 44 and set theretracted position closer to the brake friction member 20. The amount ofadjustment may be based on the difference between the estimated air gapand the desired air gap. The magnitude of the difference may then beused to determine a number of adjustment motor revolutions or length oftime to activate the adjustment motor 70 to reduce the air gap to adesired amount or distance. For example, the distance or amount of brakepad movement per revolution of the adjustment motor 70 may be calculatedor may be a predetermined amount that in turn may be used to determinehow long to activate the adjustment motor 70 or how many motor shaftrevolutions may be sufficient to reduce the air gap.

Referring to FIG. 3, a flowchart is shown that provides more detailregarding estimation of the air gap. The flowchart in FIG. 3 is bestunderstood with reference to FIGS. 4-7. In FIGS. 4-7, the rotationalposition of the operating shaft 60 detected by the actuation mechanismposition sensor 84 is plotted on the vertical axis. The vertical axismay be depicted in any suitable units that may represent angular motion,such as degrees. In FIGS. 4-7, units of measurement employed by theactuation mechanism position sensor 84 (e.g., encoder units) are used torepresent a generic unit of measure and are referenced by the letter U.Time (t) is measured along the horizontal axis. FIGS. 5-7 show thesignal from the position sensor during and after processing.

Referring to FIG. 4, a signal from the actuation mechanism positionsensor 84 before processing. From time 0 to time 1, the operating shaftmay not rotate. Thus, the actuation mechanism position sensor 84 may notdetect rotation of the operating shaft during this time period. As such,the signal may be expected to extend along the time axis (i.e., theactuation mechanism position sensor 84 may be expected to have a readingof zero); however, the signal is shown with a negative signal drift fromthe time axis (i.e., the entire signal is offset downward or negativelyfrom the expected position) which may result from one or more of thefactors previously discussed. Cycling of the friction brake is shownbetween time 1 (t₁) and time 4 (t₄). The signal may increase from time 1(t₁) to time 2 (t₂) at a first slope or with a first curvature, whichmay be indicative of actuation of one or more brake pad assembliestoward the brake friction member. From time 2 (t₂) to time 3 (t₃), thebrake pad assembly may be in engagement with the brake friction membersuch that there is no air gap. As such, it would be expected that thesignal would not change (i.e., the signal would be horizontal andconstant); however, the signal is shown with a second slope or secondcurvature which may result from signal error and/or one or more of themechanical and/or environmental factors previously discussed. The signalmay decrease from time 3 (t₃) to time 4 (t₄) at a third slope or with athird curvature, which may be indicative of disengagement of one or morebrake pad assemblies toward the brake friction member. After time 4(t₄), the actuation mechanism position sensor 84 does not detectrotation of the operating shaft.

At block 200, the method determines inflection points associated withthe signal. The inflection points may be determined by interpolation orany suitable signal processing technique. A first inflection point I₁ islocated at time 2 (t₂). The first inflection point I₁ may be indicativeof engagement of the brake pad assembly with the brake friction member.A second inflection point I₂ is located at time 3 (t₃). The secondinflection point I₂ may be indicative of release of the brake padassemblies or disengagement of the brake pad assemblies from the brakefriction member. The first inflection point I₁ may or may not have thesame magnitude as the second inflection point I₂.

At block 202, the position sensor signal may be clipped. Clipping thesignal from a position sensor may be based on the first and secondinflection points. More specifically, the signal may be clipped to thetime value that corresponds with the first inflection point I₁ and maybe clipped to the time value that corresponds with the second inflectionpoint I₂. This is best shown in FIG. 5. In FIG. 5, the signal is clippedsuch that the signal is a vertical line at time 2 (t₂) from thehorizontal line (originally shown from time 0 to time 1) to the firstinflection point I₁. Similarly, the signal is clipped such that thesignal is a vertical line at time 3 (t₃) from the second inflectionpoint I₂ to the horizontal line.

At block 204, the method may perform drift compensation. Driftcompensation may occur after clipping the signal at block 202. Driftcompensation may compensate or correct for the drift or non-zero signalthat would otherwise be expected when the friction brake is not actuatedor cycled. Drift compensation is best understood by comparing FIG. 5 toFIG. 6. In FIG. 6, the entire clipped signal in FIG. 5 is adjusted suchthat the encoder measurement is set to zero when the friction brake isnot actuated. As such, the entire position sensor signal may be adjustedupward (when there is negative signal drift) or downward (if there ispositive signal drift) so that the signal is set to zero before thefirst inflection point I₁ and/or after the second inflection point I₂.The amount of signal drift shown in FIG. 5 is designated D. Driftcompensation may be based on a drift measurement. Drift measurement maybe based on an actual or physical measurement. For example, driftcompensation may be based on drift measurement data from an absoluteposition sensor that may directly detecting or measure the width of theair gap. Alternatively, drift compensation may be based on an estimateddrift measurement provided by a model-based system in which brakepressure and or brake temperature may be used to model or determine anestimated drift amount.

At block 206, error adjustment may occur. Error adjustment may adjustthe signal to a constant value between the first inflection point I₁ andthe second inflection point I₂. Such adjustments may be based on areasoning process that may employ fuzzy logic. Error adjustment is bestshown by comparing FIGS. 5 and 6. In FIG. 5, the shaded area is theerror area. The error area may represent a signal region where a stableconstant (horizontal) signal is expected and may be bounded by thesignal located between the first inflection point I₁ and the secondinflection point I₂, or a straight line that may extend from I₁ to I₂. Acentroid or geometrical center of the error area may be determined inany suitable manner, such as by integration of the signal curve betweenand above I₁ and I₂ in the example shown in FIG. 5. The centroid may besimilar to a “center of mass” of the error area. The centroid mayinclude a time axis coordinate and an encoder axis coordinate. In FIG.5, the centroid is represented by the letter C and has an encoder axis(vertical axis) coordinate of y_(c). Encoder axis centroid and moreparticularly the encoder axis coordinate may be used to level thesignal. In FIG. 7, the signal is adjusted such that the signal is set toa constant value that corresponds to y_(c) between time 2 (t₂) and time3 (t₃) (e.g., between I₁ and I₂). After time 3 (t₃) coordinate y_(c) maybe stored or maintained until the next brake action.

The air gap may now be estimated based on the processed signal. Morespecifically, the air gap may be represented by vertical axis distanceto the constant signal between I₁ and I₂ that corresponds to y_(c). Ifdesired, this value may be converted from encoder units to other units,such as millimeters by calculation or a lookup table. For example, asingle encoder unit may correspond with a predetermined linear actuationdistance of a brake pad assembly. Such a relationship or ratio may bebased on the configuration of the brake system and/or vehicledevelopment testing.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

What is claimed is:
 1. A method of controlling a brake systemcomprising: cycling a friction brake; indirectly estimating an air gapbetween a brake friction member and a brake pad assembly based on asignal from a position sensor that detects cycling of the frictionbrake; and adjusting the air gap when an estimated air gap exceeds adesired air gap.
 2. The method of claim 1 wherein the position sensordetects rotation of an operating shaft during cycling of the frictionbrake.
 3. The method of claim 1 wherein cycling the friction brakefurther comprises actuating the brake pad assembly from a retractedposition in which the brake pad assembly is spaced apart from the brakefriction member into engagement with the brake friction member andreleasing the brake pad assembly such that the brake pad assemblydisengages the brake friction member.
 4. The method of claim 3 whereinadjusting the air gap further comprises operating an adjustment motor tochange the retracted position such that the brake pad assembly isdisposed closer to the brake friction member.
 5. The method of claim 3further comprising adjusting the air gap when an estimated air gap isless than the desired air gap by operating an adjustment motor to changethe retracted position such that the brake pad assembly is disposedfurther from the brake friction member.
 6. The method of claim 3 whereinadjusting the air gap occurs during release of the brake pad assembly.7. The method of claim 1 further comprising not adjusting the air gapwhen the estimated air gap is sufficiently close to the desired air gap.8. A method of controlling a brake system comprising: cycling a frictionbrake by actuating a brake pad assembly from a retracted position intoengagement with a brake friction member and releasing the brake padassembly such that the brake pad assembly disengages the brake frictionmember and moves toward the retracted position; indirectly estimating anair gap between the brake friction member and the brake pad assemblybased on a signal from a position sensor that is indicative of actuationof a brake pedal; and adjusting the air gap when an estimated air gapexceeds a desired air gap.
 9. The method of claim 8 wherein indirectlyestimating the air gap includes clipping the signal from the positionsensor by determining a first inflection point and a second inflectionpoint of the signal and clipping the signal to a first time value thatcorresponds to the first inflection point and to a second time valuecorresponds to the second inflection point.
 10. The method of claim 9the first inflection point is indicative of engagement of the brake padassembly with the brake friction member.
 11. The method of claim 9wherein the second inflection point is indicative of disengagement ofthe brake pad assembly from the brake friction member.
 12. The method ofclaim 9 wherein indirectly estimating the air gap further comprisesadjusting the signal to compensate for drift after clipping the signal.13. The method of claim 12 wherein indirectly estimating the air gapfurther comprises making an error adjustment by determining a centroidof an error area disposed between the first inflection point and thesecond inflection point and adjusting the signal based on the centroidof the error area.
 14. The method of claim 13 wherein making the erroradjustment includes adjusting the signal to a constant value between thefirst inflection point and the second inflection point.
 15. The methodof claim 13 wherein making the error adjustment is based on fuzzy logic.16. A brake system comprising: a brake friction member; a brake padassembly configured to move between a retracted position in which thebrake pad assembly does not engage the brake friction member and anextended position in which the brake pad assembly engages the brakefriction member; an actuator subsystem that includes an operating shaftthat rotates when the brake pad assembly moves between the retractedposition and the extended position; and a position sensor that detectsrotation of the operating shaft; wherein an air gap between the brakepad assembly and the brake friction member is estimated based on asignal from the position sensor.
 17. The brake system of claim 16wherein the signal from the position sensor is obtained when the brakepad assembly moves from the retracted position to the extended positionand back to the retracted position.
 18. The brake system of claim 16further comprising an adjustment motor, wherein the adjustment motormoves the brake pad assembly closer to the brake friction member toreduce the air gap when an estimated air gap that is based on the signalfrom the position sensor exceeds a desired air gap.
 19. The brake systemof claim 16 further comprising an adjustment motor, wherein theadjustment motor moves the brake pad assembly further from the brakefriction member to increase the air gap when an estimated air gap thatis based on the signal from the position sensor does not exceed adesired air gap.
 20. The brake system of claim 16 wherein an adjustmentmotor adjusts the air gap when the brake pad assembly moves from theextended position toward the retracted position and wherein theadjustment motor does not move the brake pad assembly from the retractedposition to the extended position.